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Old 5th March 2005, 09:16 AM   #1
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Default Looking for very simple output driver.

I'm trying to design a very simple amplifier. I want 5W output power into 4 Ohm load. I decided to use an opamp as input stage for voltage gain. But I don't know how to build an output stage that can supply enough current. I want to use mos transistors or jfets's. The less components the better and I don't mind crappy sound.

When you supply me an output stage could you explain the working to me?

I added an image for better understanding.
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Old 5th March 2005, 11:43 AM   #2
Gubby is offline Gubby  Belgium
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Default Nelson Pass ZEN ?

Why dont'you look at the Nelson Pass Zen amp ?


http://www.passdiy.com/pdf/zenamp.pdf
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Old 5th March 2005, 12:12 PM   #3
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Why not use a buffer ic ?

opamp-buffer-output stage.

A BB BUF634P you can buy at marktplaats for 2 euro.
An output stage you can build with an IRF630 and a IRF9630. and a couple of resistors.
These too can be bought at marktplaats for next to nothin.

They'll go up to 50 watts safely, and not even with crappy sound,
for like 5 euro per channel.
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Old 5th March 2005, 12:41 PM   #4
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I don't want to use the ZEN design because it has an efficiency of 20%. I want at least 60% efficiency.

I have enough BUF634's lying around here, but I don't want to use them for this project. I only want to add some capacitors, resistors and FETS/ MOSFETS.
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Old 5th March 2005, 01:48 PM   #5
joensd is offline joensd  Germany
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Some of these might be interesting:
Opamp based MOSFET amplifier design
http://www.ronybc.8k.com/mosfet.htm
http://sound.westhost.com/project76.htm

Although you can basically take any output stage you like/prefer.
Simplest approach (unless you go class A which you obviously want to avoid) would be a unity-gain-stable IC like the OPAxxx-series.
Then of course the opamp in front would be kind of "too much" really or you could switch it as input buffer as some people would advise to.
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Old 5th March 2005, 04:39 PM   #6
jcx is offline jcx  United States
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Default Re: Looking for very simple output driver.

Quote:
Originally posted by Douchekop
I'm trying to design a very simple amplifier. I want 5W output power into 4 Ohm load.

...want at least 60% efficiency.

...only want to add some capacitors, resistors and FETS/ MOSFETS.

rather tight corner you’ve painted yourself into with these requirements

5W into 4 Ohms ~ 6.3 V amplitude, ideal class B is only 70% efficient when swinging 100% of the supply voltage on the load

a 60% efficiency requirement means with a "simple" (non-switching) output your output stage must swing to less than ~700mV of the rails and the power supply must be really close to +/- 7 V

this is possible with a complementary common source output but reliable biasing of the output devices probably takes the required circuitry beyond "really simple"
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Old 5th March 2005, 04:59 PM   #7
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Can i not just throw an old philips tda at Ya ?
Ronny ridiculous, pumpin 100 watts out of two 75 watt mosfets is silly.
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